1. Field
The present invention relates generally to data communication, and more specifically to techniques for canceling interference due to pilot in a wireless (e.g., CDMA) communication system.
2. Background
Wireless communication systems are widely deployed to provide various types of communication such as voice, packet data, and so on. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), or some other multiple access technique. CDMA systems may provide certain advantages over other types of systems, including increased system capacity. A CDMA system is typically designed to implement one or more standards, such as IS-95, cdma2000, IS-856, and W-CDMA standards, all of which are known in the art.
In a wireless (e.g., CDMA) communication system, a pilot is often transmitted from a transmitter unit (e.g., a base station) to a receiver unit (e.g., a terminal) to assist the receiver unit perform a number of functions. The pilot may be used at the receiver unit for synchronization with the timing and frequency of the transmitter unit, estimation of the quality of the communication channel, coherent demodulation of a data transmission, and possibly other functions. The pilot is typically generated based on a known data pattern (e.g., a sequence of all zeros) and using a known signal processing scheme (e.g., covered with a particular Walsh code and spread with a known pseudo-noise (PN) sequence). For IS-95 and cdma2000 systems, each base station is assigned a specific offset for its PN sequence to uniquely identify its code channels from those of other base stations. In this way, the pilots from different base stations can be identified by their different assigned PN offsets.
At the receiver unit, a rake receiver is often used to recover the transmitted pilot, signaling, and traffic data. A transmitted signal may be received via multiple signal paths, and each received signal instance (or multipath) of sufficient strength may be assigned to and processed by a respective finger processor of the rake receiver. Each finger processor processes the assigned multipath in a manner complementary to that performed at the transmitter unit to recover the data and pilot in this multipath. The recovered pilot has an amplitude and phase determined by, and indicative of, the channel response for the multipath. The pilot is typically used for coherent demodulation of various traffics transmitted along with the pilot, which are similarly distorted by the channel response. The pilots for a number of multipaths are also used to combine demodulated symbols derived from these multipaths to obtain combined symbols having improved quality.
On the forward link, the multipaths from the same base station or cell (intra-cell) and the multipaths from other base stations (inter-cell) create interference when demodulating a CDMA signal. In fact, a substantial portion of the interference on any particular multipath may be from the pilots in the other multipaths included in the received signal. This pilot interference can degrade performance (e.g., higher error rate).
There is therefore a need for techniques to cancel interference due to pilot in a wireless (e.g., CDMA) communication system.
Aspects of the present invention provide techniques for canceling pilot interference in a wireless (e.g., CDMA) communication system. A received signal typically includes a number of signal instances (i.e., multipaths). For each multipath to be processed (i.e., each desired multipath), the other multipaths are effectively interfering multipaths that act as interference to the desired multipath. If the pilot is generated based on a known data pattern (e.g., a sequence of all zeros) and covered with a known channelization code (e.g., a Walsh code of zero), then the pilot in an interfering multipath may be estimated as simply the spreading sequence at a time offset corresponding to the arrival time of that multipath at the receiver. The pilot interference from each interference multipath may be estimated based on the spreading sequence for the interfering multipath and the despreading sequence for the desired multipath. The total pilot interference from a number of interfering multipaths may be subtracted from the data component (or data signal) in the desired multipath to provide pilot-canceled data having improved performance.
In one specific embodiment, a method is provided for canceling pilot interference at a receiver unit that receives first and second pilot signals and first and second data signals. In accordance with the method, a first PN sequence corresponding to the first pilot signal is generated and despread with a second PN sequence corresponding to the second pilot signal. The despread first PN sequence is then multiplied with an estimated gain and phase of the first pilot signal to generate a first multiplied signal, which represents the estimated interference from the first pilot signal on the second data signal. The first multiplied signal is then subtracted from the second data signal. Similar processing may be performed for the second pilot signal and first data signal.
Various aspects, embodiments, and features of the invention are described in further detail below.